Myosin is believed to generate force and movement by the rotation of a long a-helical region that extends from the C-terminus of the motor domain, and is stabilized by the essential light chain (ELC) and the regulatory light chain (RLC). The role of the light chain-binding domain or "lever arm" is to amplify small conformational changes originating at the nucleotide binding site into larger movements of the lever arm. Despite recent advances in kinetic and structural approaches, many aspects of the communication pathway between ATP hydrolysis, actin-binding, and the lever arm remain unresolved. This is due, in part, from an absence of structural information regarding the flexible N-terminal regions of the light chains, and the actin- myosin interface for which no atomic structure exists. Here we propose advanced techniques in electron cryomicroscopy (cryoEM) and image analysis, fluorescence microscopy and transient kinetics to provide further insights into the mechanism of mechanochemical coupling. Specific Aim 1 will examine the binding of the N-terminal extension of ELC to the SH3 (src-homology 3) domain. To date, the function of the SH3-like (3-barrel domain in myosin is unknown. We will test the hypothesis that SH3 mediates the communication pathway between the ELC-1 isoform, actin, and the catalytic site, by using gold-labeled- (for cryoEM) and fluorescent-labeled (for spectroscopy) mutants of expressed ELC. Any conformational changes in ELC will be correlated with steps in the ATPase cycle by stopped-flow kinetics. Aim 2 will examine how phosphorylation of the RLC leads to activation of smooth muscle myosin from its inhibited, dephosphorylated state. The hypothesis that the N-terminus undergoes a major conformational change will be tested by introducing labeled cysteine residues into RLC and ELC to facilitate determination of length changes and sites of interaction by fluorescence spectroscopy. Aim 3 will characterize the actomyosin interface by high resolution cryoEM, and new computational methodologies, using actin filaments decorated with wild type and mutant cardiac myosin isoforms. Pathophysiological conditions involving abnormal expression of the light chains, as well as cardiomyopathies resulting from point mutations in the light and heavy chains of myosin, will benefit from a deeper understanding of how the different light chains and domains in myosin interact to perform work with maximum contractile efficiency.